The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for sequentially alternating among plasma processes in order to optimize a substrate.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited.
Generally, process variables are often adjusted in order to main acceptable plasma process characteristics, such as etch rate, uniformity, selectivity, etch profile, etc. Etch rate is the measure of how fast material is removed in the etch process. It is an important characteristic of the process, since it directly affects the throughput of the etch process. The etch rate can be calculated by measuring the film thickness before and after the etch process and dividing the thickness difference by the etch time.
Uniformity is the degree of etch rate consistency across a substrate surface. It is normally measured by thickness at certain points before and after the etch process, and calculating the etch rates at these points. Selectivity is the ratio of the etch rates between the different materials, especially the material that needs to be etched compared with the material that should not be removed. Profile is the degree of vertical-ness of an etch. Generally, the greater in surface uniformity of a feature wall, the better the profile.
Among the set of process variables that can be adjusted are the process time, RF power, chamber pressure, gas composition, gas flow, substrate bias, RF frequency, etc. However, although in theory it may be beneficial to optimize each variable for each plasma process characteristic, in practice it is often difficult to achieve. Very often optimum process conditions can only be maintained within a narrow window of parameters, and hence not practical from a manufacturability standpoint. By adjusting one variable to improve one characteristic, another characteristic may deteriorate.
In general, there are three types of etch processes used to etch the various layers on the substrate: pure chemical etch, pure physical etch, and reactive ion etch.
Pure chemical etching generally involves no physical bombardment, but rather a chemical interaction of neutral molecules (neutrals) with materials on the substrate (e.g., Al, etc.). Subsequently, the chemical reaction rate could be very high or very low, depending on the process. For example, fluorine-based molecules tend to chemically interact with dielectric materials on the substrate, wherein oxygen-based molecules tend to chemically interact with organic materials on the substrate, such as photoresist.
Pure ion etching, often called sputtering, is used to dislodge material from the substrate (e.g., oxide, etc.). Commonly an inert gas, such as Argon, is ionized in a plasma and subsequently accelerate toward a negatively charged substrate. Pure ion etching is both anisotropic (i.e., principally in one direction) and non-selective. That is, selectivity to a particular material tends to be very poor, since sputtering rate of most materials are similar. In addition, the etch rate of the pure ion etching is commonly low, depending generally on the flux and energy of the ion bombardment.
Reactive ion etch (RIE), also called ion-enhanced etching, combines both chemical and ion processes in order to remove material from the substrate (e.g., photoresist, BARC, TiN, Oxide, etc.). Generally ions in the plasma enhance a chemical process by striking the surface of the substrate, and subsequently breaking the chemical bonds of the atoms on the surface in order to make them more susceptible to reacting with the molecules of the chemical process. Since ion etching is mainly perpendicular, while the chemical etching is both perpendicular and vertical, the perpendicular etch rate tends to be much faster than in then horizontal direction. In addition, RIE tends to have an anisotropic profile.
However, one problem that has been encountered with both pure chemical etching and RIE etching has been a non-uniform etch rate. Etch rate is generally the measure of how fast material is removed in the etch process. It is generally calculated by measuring the thickness before and after the etch process and dividing the thickness difference by the etch time.
In general, the etch rate is typically higher at the edge of the substrate where the local etch rate may be dominated by either chemical reactions at the surface, or by limited etchant transport to the substrate surface. That is, since less substrate surface area is available to etch for a given volume of etchant, a greater etch rate tends to result.
Referring now to FIG. 1, a simplified diagram of plasma processing system components is shown. Generally, an appropriate set of gases is flowed into chamber 102 through an inlet 109 from gas distribution system 122. These plasma processing gases may be subsequently ionized at injector 108 to form a plasma 110, in order to process (e.g., etch or deposition) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned with edge ring 115 on an electrostatic chuck 116. In addition, liner 117 provides a thermal barrier between the plasma and the plasma processing chamber, as well as helping to optimize plasma 110 on substrate 114.
Induction coil 131 is separated from the plasma by a dielectric window 104, and generally induces a time-varying electric current in the plasma processing gases to create plasma 110. The window both protects induction coil from plasma 110, and allows the generated RF field to penetrate into the plasma processing chamber. Further coupled to induction coil 131 at leads 130a-b is matching network 132 that may be further coupled to RF generator 138. Matching network 132 attempts to match the impedance of RF generator 138, which typically operates at 13.56 MHz and 50 ohms, to that of the plasma 110.
Referring now to FIG. 2, a simplified view of an etch process is shown. In general, a plasma etch process is substantially complex, and influenced by many factors. For example, an RF field creates several types species in plasma 110, such as high energy electrons, positive ions, negative ions, neutrals, and radicals. Positive ions are created when an electron is completely removed from a gas molecule or atom. Likewise, negative ions although rare, are created when an electron is added to a gas molecule or atom.
Radicals are created when electron collisions break up molecules into fragments which as a result have unsatisfied chemical bonding and are chemically reactive. Since they have no net charge, and therefore are not accelerated by the field or are not attracted by charged particles, they tend have a long lifetime compared to charged particles. Neutrals are stable, having neither a positive nor negative charge, nor are chemically active. Generally, two of the most important parameters are the number density and energy distribution of the electrons, which play a central role in initiating and maintaining the plasma.
In general, in a plasma etch process, directional etching is achieved by sidewall passivation, often through polymer formation 224 on the etch front. The amount of sidewall passivation depends on the amount of etch product and mask area, and it changes dramatically as one moves from isolated features to densely populated portions of the integrated circuit. The amount of sidewall passivation material determines the profile of the structure.
Some of the reactants in the plasma are transported to the substrate surface 202, where reaction 201 may occur, such as physi-sorption or chemisorption 204. In chemisorption, a strong “chemical bond” is formed between the adsorbed atom or molecule and the substrate. Physisorption is weaker, and is often being considered as having no chemical interaction involved.
Other reactants may then be transported to etch front 214, or deflected away if composed of ions 208. In combination with substrate temperature control 216 and bias creation 218, these factors may subsequently affect profile 210 and surface quality 212. As previously described, ions are often used in etch reaction 220 to physically dislodged material from the substrate (e.g., oxide, etc.), while neutrals and radicals may be used in a chemical etch reaction 220. Reaction by-products often diffuse back into the main plasma gas stream and may be subsequently pumped 228 from plasma chamber 230.
Referring now to FIG. 3, a simplified diagram comparing etch rate to RF power in a plasma etch process. All other plasma process characteristics held constant, at low RF power, fewer ions may be available in the plasma to enable the chemical process by striking the surface of the substrate. As RF power increases, additional ions are created, enabling the overall etch rate to increase. However, increasing the RF power still further stabilizes the etch rate, since the plasma, saturated with ions, has a smaller mean free path. Mean free path (MFP) may be defined as the average distance that the ion can travel before it collides with another particle. In general, the shorter the MFP, the smaller the amount of ions that reach the surface of the substrate.
Referring now to FIG. 4, a simplified diagram comparing etch rate to pressure in a plasma etch process is shown. In contrast to FIG. 3, all other plasma process characteristics held constant, in general, for physically dominant etching, increasing pressure will tend to reduce mean free path, and hence the number of ions available for etching. That is, by increasing pressure, more ion collisions result in decreased ion energy.
Referring now to FIG. 5, a simplified diagram comparing uniformity to RF power in a plasma etch process is shown. All other plasma process characteristics held constant, in general, at low RF power, fewer ions may be available in the plasma to enable the chemical process by striking the surface of the substrate. As RF power increases, additional ions are created, enabling the overall etch rate to increase. However, as there is more etchant available at the edge of the substrate than at the center, the edge etch rate tends to be greater than the center etch rate, decreasing the substrate uniformity. Increasing the RF power still further saturates the plasma with ions, reducing mean free path.
Referring now to FIG. 6, a simplified diagram comparing uniformity to pressure in a plasma etch process is shown. In contrast to FIG. 5, all other plasma process characteristics held constant, in general, initially increasing pressure also increases the etchant transport to the substrate surface, equalizing the etch rate between the edge and center of the substrate. However, as before, increasing pressure still further saturates the plasma with ions which also reduces mean free path, and hence the amount of ions available to reach the surface of the substrate.
Referring now to FIG. 7, a simplified diagram comparing selectivity to RF power in a plasma etch process is shown. All other plasma process characteristics held constant, in general, increasing RF power reduces etch selectivity since the etch process tends to be more physical (i.e., sputtering) and less chemical. As previously described, selectivity is the ratio of the etch rates between the different materials, especially the material that needs to be etched compared with the material that should not be removed.
Referring now to FIG. 8, a simplified diagram comparing selectivity to pressure in a plasma etch process is shown. In contrast to FIG. 7, all other plasma process characteristics held constant, in general, increasing pressure tends to increase selectivity, since the plasma, saturated with ions, has a smaller MFP. Since fewer ions are available to reach the substrate surface, the etch process tends to be less physical and more chemical.
Referring now to FIG. 9 a simplified diagram comparing vertical-ness of etch profile to RF power in a plasma etch process is shown. All other plasma process characteristics held constant, in general, increasing RF power tends to increase the number of ions and hence the vertical etch rate and subsequent vertical profile. As previously described, ion etching tends to be mainly perpendicular to the substrate while the chemical etching is both perpendicular and vertical.
Referring now to FIG. 10 a simplified diagram comparing vertical-ness of etch profile to pressure in a plasma etch process is shown. In contrast to FIG. 9, all other plasma process characteristics held constant, in general, as pressure is increases, the vertical-ness of the etch profile decreases, since the plasma, saturated with ions, has a smaller MFP. Since fewer ions are available to reach the substrate surface, the etch process less physical and more chemical.
In view of the foregoing, there are desired improved methods and apparatus for sequentially alternating among plasma processes in order to optimize a substrate.